1. Field of the Invention
The present invention relates to a radiocommunication apparatus, a radiocommunication system, and a radiocommunication method which are preferably adapted to, for example, a radio network system to be installed on the premises.
2. Description of the Related Art
With the advancement of computers, it has become a boom to interconnect a plurality of computers to construct a local area network (LAN) for the purpose of utilizing files or data in common or transferring electronic mails or data. Existing LANS are constructed with computers interconnected over an optical fiber, a coaxial cable, or a twisted-pair cable.
Such a wire LAN requires construction work for connections and is therefore hard to construct. Moreover, cables employed are rather annoying. A wireless LAN is therefore attracting people""s attention as a system relieving a user from cable connection needed to construct the conventional wire LAN.
A wireless LAN over which data is communicated according to the code division multiple access (CDMA) method based on the modulation method of spectrum spreading has been proposed. According to the CDMA method, communication data is multiplied based on a pseudonoise (PN) code and the spectrum of a signal representing the data is thus spread. The data transmitted through spectrum spreading is demodulated while being multiplied based on the same PN code as that used on the transmitting side. The CDMA method is characterized in that it can highly reliably maintain privacy and withstand interference.
In recent years, a multimedia approach to information has made a progress and a large amount of data including image data and voice data has come to be treated. The wireless LAN is requested to offer a high transfer rate so that a large amount of data including image data and voice data can be transferred. However, according to the modulation method of spectrum spreading, when an attempt is made to transfer data at a high transfer rate of, for example, 30 Mbps, a frequency band of 300 MHz and above is needed. Such a wide frequency band cannot be preserved according to an existing standard on frequency assignment. It is impossible to preserve the wide frequency band for communication.
Moreover, according to the spectrum spreading method, synchronism must be attained so that a signal representing a code of transmitted data will be in phase with a signal representing a code generated by a receiver for demodulation. Therefore, according to the spectrum spreading method, a bit stream intended for synchronization is inserted into each packet. This poses a problem that the bit stream for synchronization leads to an increased number of bits other than those serving as effective data.
The present invention attempts to solve the foregoing problems underlying the related art. An object of the present invention is to provide a radiocommunication apparatus, a radiocommunication system, and a radiocommunication method capable of transmitting a large amount of data including image data and voice data at a high rate, and helpful in decreasing the number of bits other than bits serving as effective data.
Moreover, the present invention attempts to break through the current situation of a background art to be described below. Another object of the present invention is to provide a radiocommunication apparatus, a radiocommunication system, and a radiocommunication method in which a receiving side can reliably detect synchronism attainment signals under a multipath environment, and highly precisely demodulate a signal.
The Inventor has proposed a technique of transmitting payload type data according to the orthogonal frequency division multiplexing (OFDM) method and communicating data in units of a frame according to the time division multiple access (TDMA) method. Herein, a synchronism attainment signal representing a predetermined pattern of bits such as a maximum-length shift-register sequence is transmitted within the leading time interval of one frame. The timing of transferring payload type data is determined with the synchronism attainment signal as a reference. According to the technique, a receiving side detects the synchronism attainment signal using a matched filter or the like, and determines the timing of demodulating the payload type data present within the frame according to the detected timing of the synchronism attainment signal. Consequently, a symbol serving as the data present within the frame and transmitted according to the OFDM is demodulated accurately.
FIG. 18 shows the configuration of a reception circuit 500 serving as the background of the present invention proposed by the Inventor. FIG. 19 is a flowchart describing processing to be performed by the reception circuit 500 shown in FIG. 18. In the reception circuit 500 shown in FIG. 18, a reception signal S501 received by an antenna 501 is converted into a radio-frequency signal by a radio-frequency conversion circuit 502 and digitized by an A/D converter 503. This results in a reception signal S503.
The reception signal S503 is output to a cross-correlation detection circuit 504 and an OFDM modulation circuit 507.
The cross-correlation detection circuit 504 uses, for example, a matched filter to detect the cross-correlation of the reception signal S503 relative to a synchronism attainment signal representing a predetermined pattern of bits that is acquired in advance. Consequently, a cross-correlation signal S504 representing the detected cross-correlation is output to a demodulation timing determination circuit 506 (step S1).
The cross-correlation signal S504 exhibits a pulse abruptly at the timing corresponding to the timing of the synchronism attainment signal contained in the reception signal S503.
The demodulation timing determination circuit 506 compares the level of the cross-correlation signal S504 with a predetermined threshold, and thus detects the timing at which the level of the cross-correlation signal S504 exceeds the threshold (step S2).
The demodulation timing determination circuit 506 produces a demodulation timing signal S506 that exhibits a pulse at the detected timing, and outputs the demodulation timing signal to the OFDM demodulation circuit 507 (step S3).
The OFDM demodulation circuit 507 demodulates a payload type signal at the timing determined with the demodulation timing signal S506 as a reference (step S4).
According to the background art proposed by the Inventor, the level of the cross-correlation signal S504 may not, as shown in FIG. 20B, exceed the threshold at the timing of a sample thereof coincident with the synchronism attainment signal under a multipath environment. Otherwise, the level of the cross-correlation signal S504 may exceed the threshold at the timing of a sample thereof other than the sample coincident with the synchronism attainment signal. Herein, the multipath environment is a condition that radio waves are propagated from a transmitting side within a communication system to a receiving side over a plurality of paths because the radio waves are reflected from structures within a building or on the premises. This poses a problem in that the demodulation timing determination circuit 506 may not be able to accurately produce the demodulation timing signal S506.
FIG. 20A shows an example of the waveform of the cross-correlation signal S504 under a non-multipath environment. FIG. 20B shows an example of the waveform of the cross-correlation signal S504 under the multipath environment. In FIG. 20A and FIG. 20B, the axis of abscissas indicates the timing of a sample, and the axis of ordinates indicates the level of the cross-correlation signal S504.
For overcoming the drawbacks of the aforesaid related art and background art, the present invention provides constituent features and operations described below.
According to the first aspect of the present invention, a radiocommunication apparatus attains the synchronism of a reception signal, which contains a plurality of synchronism attainment signals representing the same pattern of bits, relative to a reference signal. The radiocommunication apparatus consists mainly of an auto-correlation detection circuit, a cross-correlation detection circuit, and a synchronism attainment circuit. The autocorrelation detection circuit detects the autocorrelation function of the reception signal. The cross-correlation detection circuit detects the cross-correlation function of the reception signal. The synchronism attainment circuit detects the synchronism attainment signals contained in the reception signal according to the detected autocorrelation function and cross-correlation function, and attains the synchronism.
Operations to be exerted by the radiocommunication apparatus in accordance with the first aspect of the present invention will be described below.
The autocorrelation detection circuit detects the autocorrelation of the reception signal.
The cross-correlation detection circuit detects the cross-correlation of the reception signal.
The synchronism attainment circuit detects the synchronism attainment signals contained in the reception signal according to the detected autocorrelation function and cross-correlation function, and attains the synchronism according to the detected synchronism attainment signals.
According to the second aspect of the present invention, a radiocommunication apparatus attains the synchronism of a reception signal, which contains a plurality of synchronism attainment signals representing the same pattern of bits, relative to a reference signal. The radiocommunication apparatus consists mainly of an autocorrelation detection circuit, a cross-correlation detection circuit, a mask circuit, and a synchronism attainment circuit. The autocorrelation detection circuit detects the autocorrelation function of the reception circuit. The cross-correlation detection circuit detects the cross-correlation function of the reception signal. The mask circuit masks a predetermined range of the detected cross-correlation function covering the cross-correlation functions of the synchronism attainment signals according to the detected autocorrelation function. The synchronism attainment circuit detects the synchronism attainment signals contained in the reception signal according to the masked cross-correlation function, and thus attains the synchronism.
Operations to be exerted by the radiocommunication apparatus in accordance with the second aspect of the present invention will be described below.
The autocorrelation detection circuit detects the autocorrelation function of the reception signal.
The cross-correlation detection circuit detects the cross-correlation function of the reception signal.
The mask circuit masks the predetermined range of the detected cross-correlation function covering the cross-correlation functions of the synchronism attainment signals according to the detected autocorrelation function.
The synchronism attainment circuit detects the synchronism attainment signals contained in the reception signal according to the masked cross-correlation function, and thus attains the synchronism.
In the radiocommunication apparatus in accordance with the second aspect of the present invention, preferably, the synchronism attainment circuit compares the masked cross-correlation function with a predetermined threshold to detect the synchronism attainment signals.
In the radiocommunication apparatus in accordance with the second aspect of the present invention, preferably, the synchronism attainment circuit specifies a maximum value of the masked cross-correlation function to detect the synchronism attainment signals.
In the radiocommunication apparatus in accordance with the second aspect of the present invention, preferably, the synchronism attainment circuit judges whether the masked cross-correlation function exceeds a predetermined threshold. The synchronism attainment circuit specifies a maximum value from among all the values of the cross-correlation function exceeding the threshold, and thus detects the synchronism attainment signals.
Moreover, in the radiocommunication apparatus in accordance with the second aspect of the present invention, preferably, the synchronism attainment signals are signals representing data encoded according to a pseudo spreading code.
Moreover, in the radiocommunication apparatus in accordance with the second aspect of the present invention, preferably, the synchronism attainment signals are signals transmitted according to the OFDM method.